Besides a power engine and a transmission, the drivetrain of a motor vehicle has a clutch positioned between the power engine and the transmission so that when the clutch is disengaged, the power engine is decoupled from the transmission and when the clutch is engaged, the power engine is coupled to the transmission. The present invention concerns a method for the automatic actuation of a clutch of a drivetrain, in which the transmission of the drivetrain is an automatic transmission and the clutch is an automatically actuated clutch.
When a motor vehicle is rolling down a slope with the accelerator pedal not actuated, the clutch should be engaged so far as possible such that the braking effect of the power engine can act upon the motor vehicle. Moreover, when rolling downhill with the clutch engaged a so-termed throttle cut-off of the power engine can be activated, whereby fuel can be saved. In contrast, when the motor vehicle is coasting on level ground with the accelerator pedal not activated, the clutch should be disengaged so far as possible so that the motor vehicle can continue coasting. Accordingly, a method for actuating a clutch of a drivetrain when the accelerator pedal is unactuated must cover the situations when the motor vehicle is rolling downhill and the clutch has to absorb the traction torque of the power engine and must therefore not separate, and when the motor vehicle is coasting, the clutch must separate far enough to allow coasting without so-termed drag.
In practice the clutch position of the drivetrain's clutch is determined as a function of a rotation speed difference between a transmission input speed and an idling speed of the power engine. Particularly when the speed difference is very small, i.e. when the transmission output speed is about the same as the power engine's idling speed, the determination of an appropriate clutch position on the basis of the rotation speed difference presents difficulties. This can result is the clutch being engaged and disengageded several times one after another, which clearly has an adverse effect on comfort.